Integrated circuits (ICs) using CMOS techniques are susceptible to single-event upsets due to alpha particles. An alpha particle can cause ionizing radiation when it passes through a semiconductor device. The resulting charge generated by an alpha particle can accumulate at a device node and change the state of the node, typically by shorting a transistor source-drain and hence disturbing the logic state of that transistor. For example, if a node in a memory cell stores a zero data value, charge accumulation at the node can flip the data value to a one. Such data state changes are commonly called “single-event upsets” (SEUs).
A common source of alpha particles is the material used in fabricating ball grid arrays (BGAs) or bump arrays on integrated circuits. Bump arrays and BGAs are often used to provide electrical and mechanical connections between an IC chip and a printed wiring board or package carrier. The material used to form the balls or bumps often contains lead, which can be a source of alpha particles.
Various techniques have been developed to avoid alpha particles emitted by lead in solder bumps from creating SEUs in the associated IC. One approach is to coat the solder bumps with a layer of alpha particle absorbing material. Another approach incorporates a high-density layer of alpha particle absorbing metal deposited on selected areas of the IC. Both of these approaches introduce additional process steps into the IC manufacturing sequence and hence add additional undesired cost.
Techniques that reduce SEUs due to alpha particles from balls or bumps on ICs that avoid the disadvantages of the prior art are desirable.